This invention relates to the field of seismic data acquisition and processing.
Converted-wave (C-wave) processing is more difficult than pure-mode processing. Those of the ordinary skill in the art will recognize that converted waves result from compressional waves (P-waves), which come into contact with a reflecting surface and incite a shear wave (S-wave). The opposite is also true. That is, an S-wave may also incite a P-wave. Because P-waves and S-waves have different characteristics, converted wave processing, which requires a set of characteristics for the down-going wave and a different set of characteristics for the up-going wave, is much more complicated than processing only a single set of characteristics for only P-wave data or S-wave data.
For example, accurate migration is critically dependent on both P-wave and S-wave velocities. Furthermore, conventional prestack time migration techniques, such as the industry standard of Kirchhoff migration, are normally based on the double-square root travel time equation. These methods assume a straight ray from the source to the scatter point and from the scatter point to the receiver. Other current Kirchhoff prestack time migration techniques are often implemented with a travel time approximation (Li et al., 2001). The straight ray assumption and the travel time approximation thus, limit the ability to produce an accurate subsurface image without a correction for ray bending and travel time errors. For multi-layered v(z) media or vertical transversely isotropic (VTI) media, more sophisticated travel time equations are needed.
Even further, another current prestack migration is equivalent-offset migration. Equivalent-offset migration (EOM) is a prestack time migration method proposed by Bancroft et al. (1998). Equivalent offset migration claims to be computationally efficient and weakly velocity-dependent. However, in C-wave processing, equivalent offset migration (Wang et al, 1996) depends on the initial velocity even for a single constant-velocity flat layer. Thus, EOM is poor for C-wave velocity analysis.
Finally, many current ocean bottom cable (OBC) techniques require wave-equation datumming. Wave equation datumming adds additional processing steps, increases cost, and causes irregular geometry effects on 3-D OBC data processing.
Thus, there is a long felt need for migration techniques which address one or any of the above issues.